Liquid handling systems for moving and storing liquids generally require a pump for moving liquid from one location to another and means for determining the level of a liquid in a tank or other liquid storage vessel. One such liquid handling system is a condensate pump for use with a heating, ventilation, and air-conditioning (HVAC) system. A conventional condensate pump has a tank or reservoir for collecting condensate from the evaporator of the HVAC system, and a centrifugal pump for pumping the condensate liquid from the tank to a remote location for disposal. The centrifugal pump may be submerged in the liquid inside the tank or may be located outside the tank, typically in a location that is lower than the liquid level in the tank.
When the centrifugal pump is submerged in the liquid, the centrifugal pump is positioned at the lowest point in the tank in order to assure that the centrifugal pump can remove most of the liquid from the tank. An electric motor is typically mounted above the tank and is connected to the impeller of the centrifugal pump by means of a shaft. Likewise, the control circuitry is typically mounted adjacent the motor. The electric motor spins the impeller within a volute-shaped housing of the centrifugal pump, and through centrifugal force, the impeller expels the liquid from the volute-shaped housing through one or more pump outlets that are tangent to the impeller's direction of rotation. The centrifugal pump may be plumbed to convey the liquid from the pump outlet to an elevation higher than that of the tank. Often the plumbing circuit connected to the pump outlet includes a check valve to prevent liquid from flowing back into the tank when the pump is shut off. In order to control the operation of the motor and therefore the operation of the centrifugal pump, the control circuitry must include means for determining the level of liquid in the tank. Such means for determining the level of liquid in the tank may include mechanical means, such as floats, or may include electric means, such as capacitance plates submerged in the liquid in the tank.
In condensate pumps where the centrifugal pump is positioned below the tank, the bottom of the tank may be fitted with a drain or screen-drain, the location of which is at the lowest point of the tank to receive the liquid by way of a gravity feed. The drain fitting is plumbed to the inlet of the centrifugal pump to convey the liquid to the pump's impeller.
The centrifugal pump system described above, whether submerged in the liquid or connected to a drain from the tank, may be plagued with difficulties as a result of air or other gas trapped inside the volute-shaped housing of the centrifugal pump. Once the tank is filled with liquid, the centrifugal pump must start against head pressure created by liquid located above the pump in the tank and in the pump's outlet plumbing. As long as the volute-shaped housing is filled with liquid, the pump can start, expel liquid, and draw in new liquid from the tank. A problem may occur if the liquid has entrained air or gas. On the suction side of the pump (the pump inlet), trapped gas will tend to expand and separate from the liquid. This trapped gas, being less dense than the liquid, will be forced away from the outlet of the pump by the denser and higher pressure at the impeller's periphery, and the trapped gas will tend to collect at the suction or neutral pressure center of the impeller. If the fluid flow is great enough, the trapped gas will be expelled through the pump outlet along with the liquid. Consequently, the centrifugal pump can be caught in three distinct modes of operation:                1. In a normal pumping mode, the liquid completely fills the inlet and outlet of the centrifugal pump, and the centrifugal pump continuously intakes liquid through the pump inlet and expels the liquid through the pump outlet.        2. In a second pumping mode of operation, gas expands out of the inflowing liquid, creates gas bubbles inside the volute-shaped housing, and minor cavitation results during the pumping operation. Because of the low volume of gas, some liquid flow continues, and the gas is discharged through the outlet of the volute-shaped housing. In this situation, pumping efficiency is reduced and audible noise is increased because of the cavitation.        3. In a third pumping mode, gas expands out of the inflowing liquid and creates a gas bubble at the inlet of the centrifugal pump. Liquid trapped at the discharge outlet of the pump and around the periphery of impeller creates a high pressure restriction. Between the liquid head pressure from the tank and the liquid head pressure of the outlet discharge plumbing, the centrifugal pump cannot move the gas bubble that is trapped in the pump's volute-shaved housing. Because the gas bubble cannot be cleared from the volute-shaped housing, liquid flow does not occur, and the pump simply spins gas or a gas/liquid mixture. This condition is sometimes confused with cavitation but in fact is a simple balance of liquid pressure and gas pressure within the volute-shaped housing. Often the bubble of gas will remain in the impeller's volute-shaped housing when the centrifugal pump is stopped and will continue to block liquid flow through the pump when the pump is restarted.        
As previously indicated, in order to control the operation of the centrifugal pump for a condensate pump, the condensate pump must be able to determine accurately the liquid level in the tank and in the pump's volute-shaped housing. In a conventional condensate pump, a float monitors and detects the water level within the pump's tank. In response to movement of the float within the tank, associated float switches and a float control circuitry control the operation of the electric motor driving the impeller of the centrifugal pump, trigger alarms, or shut down the HVAC system if necessary. The condensate pump float is in contact with the water in the tank and is subject to fouling from debris and algae buildup. A molded float has seams, which may fail causing the float to sink or malfunction. The float switch that is used to control the on/off operation of the electric motor is often a specialized and costly bi-stable snap-action switch. A conventional condensate pump, which incorporates a safety HVAC shut off switch and/or an alarm switch in addition to the motor control switch, may have a separate float or linkage to operate the HVAC shutoff switch or the alarm switch further complicating the condensate pump. Further, a conventional condensate pump often requires a float mechanism retainer to prevent shipping damage, and the float mechanism retainer must be removed prior to pump use.
The prior art has also adopted capacitive sensors as liquid level detectors to determine the level of the water in the tank of the condensate pump to replace the mechanical float for controlling the operation of the pump motor, for triggering alarms, or for shutting down the HVAC system if necessary. In some conventional liquid level detectors, at least one of the capacitance plates of the capacitive sensors is in contact with the water in the tank in order to produce a detectable change in capacitance as the water contacts or exposes the capacitance plate of the capacitive sensor. In another prior art capacitive sensor, the capacitance plates are mounted outside of the tank and not in contact with the water in the tank. In order to determine accurately the water level, such prior art external capacitive sensors have a first capacitance plate extending the height of the tank and one or more additional capacitance plates position at anticipated transition points along the height of the tank in order to determine when the water level has reached one of the transition points. Such additional capacitance plates are deemed necessary in order to offset the effects of deposits that may form on the inside of the tank adjacent to the external capacitive sensor thereby affecting the capacitance value.